sf_soft: Various fixes and modifications

Support only one argument in sf_soft_exists(), sf_soft_is _installed(), sf_soft_is_upgradeable(), sf_soft_is_up_to_date(), sf_soft_available_version()
Improve and fix documentation
Rewrite sf_soft_available_version() because 'yum check-update' displays confusing messages when a package is obsoleted by another one. 'yum list available' result is easier to parse.
Fix TODO file

Project todo list is now maintained using github issue manager
Switch version to 1.1.25
Merge branch 'release_1.1.25' into develop
Add sf_soft_repo_list() function